1. Field of the Invention
The present invention relates to a semiconductor device such as a thin film transistor (hereinafter referred to as TFT) which is formed on a substrate using a crystalline semiconductor film as well as to a manufacturing method of the same. The semiconductor device of the present invention includes not only elements such as TFT and MOS transistor, but also a liquid crystal display device comprising a semiconductor circuit (micro-processor, signal processing circuit and high frequency circuit and the like) constructed of the above insulation gate type transistors, an EL (Electro Luminescence) display device, an EC (Electro Chromic) display device and an image sensor and the like.
2. Description of the Prior Art
At present, as a semiconductor element using a semiconductor film, a thin film transistor (hereinafter, referred to as TFT) is used in a variety of integrated circuits, in particular, as a switching element of an image display device. In addition, a TFT using a crystalline semiconductor film, of which the mobility is higher than that of an amorphous semiconductor film, as an active layer has a high driving performance and is used as an element of a driving circuit.
Reliability is seen as the most important factor in evaluating a TFT. As for a factor that decreases reliability, impurities in a TFT (hereinafter, impurities which lower the reliability of a TFT are referred to as contaminant impurities in the present specification) are cited. These contaminant impurities become mixed into a TFT from a variety of contamination sources, such as an atmosphere, a glass substrate or a manufacturing device, etc. In particular, the existence of contaminant impurities on the interfaces of a film forming a TFT becomes a major factor that decreases the reliability of the TFT.